In currently developed 3-DOF (Degree of Freedom) translational manipulators, high output stiffness and accuracy are limited by their respective topological structures. The general topological structural forms for such manipulators include serial and parallel forms. The serial forms of 3-DOF translational manipulators are opened-loop structures. Errors of the output motions are accumulated by the serial connections, and the stiffness, load/weight ratio, and speed of the serial manipulators are limited. In order to achieve higher stiffness, accuracy, load/weight ratio and operation speeds of translational manipulators, closed-loop parallel forms of 3-DOF translational manipulators can be used to simultaneously constrain the output motions generated by three serial kinematic chains. However, the developed 3-DOF translational parallel manipulators have respective structural disadvantages, such as moving actuators, independent actuation forces, anisotropic constraints, singularity configurations and complex joint combinations.
In the development of parallel manipulators, manipulators with a limited number of parallel DOF have been designed for many different industrial applications because 6-DOF motions are not always required. Compared to serial manipulators, the rotational ability of the developed parallel manipulators is limited by multi-loop constraints. Thus, the designs of limited-DOF parallel manipulators are focused on translational output motions, while the required rotational output motions are realized by mounting serial revolute joints on the output movable platform of the parallel manipulator.
Some of the drawbacks of current manipulators include, but are not limited to: Having too many 1-DOF joints included in the structure of the parallel manipulators; low stiffness and accuracy are induced by cumulative effects of the joints; some designs have orthogonal constraints; having three revolute joints in each subchain may be responsible for low stiffness and accuracy of the output movable platform; the prismatic joints, placed on the movable platform of certain designs, require a large-sized movable platform to increase a range of the workspace; a low load/weight ratio is inevitable with large-sized movable platforms; planar joints are not easy to fabricate and depend on the design of planar aerostatic bearing systems; constraint forces of the planar joints are not uniform when heavy loads are applied to the movable platform; etc.
Three typical 3-DOF TPMs include the Delta Robot, the Tsai mechanism and the Cartesian parallel manipulator. These designs have been used in many practical applications including manufacturing, medical and food industries to mention a few. However, these developed 3-DOF TPMs still suffer from a variety of drawbacks. The structural disadvantage of the Delta Robot is its complex structure with a total of twenty-one 1-DOF joints. The Tsai mechanism includes six universal joints, and the three actuators are not placed on the fixed platform. Cartesian parallel manipulators have orthogonally arranged constraints and, furthermore, the actuation force of each subchain independently resists the external force. All of the above-mentioned characteristics lead to low stiffness and accuracy of these parallel manipulators.
In light of the above, a need exists to develop a translational parallel manipulator that overcomes the above-identified and other disadvantages.